1. Field of the Invention
The present invention relates to a vehicle front and rear wheels drive system and a clutch changeover method.
2. Description of the Related Art
In conventional four-wheel drive vehicles (hereinafter, referred to as xe2x80x9c4WD vehiclesxe2x80x9d), when turning a corner having a small turning radius at low to intermediate speeds in four-wheel drive running conditions, there occurs a tight corner braking phenomenon due to the generation of a turning speed difference between front and rear wheels attributed to a difference in turning radius. Generally known as a technology for solving the problem of the tight corner braking phenomenon is a front and rear wheels drive system such as disclosed in JP-B-7-64219. The front and rear wheels drive system so disclosed is such that the average wheel speed of the secondary drive wheels is adjusted relative to the average wheel speed of the primary drive wheels by providing a change-speed apparatus between the primary drive wheels and the secondary drive wheels. Specifically speaking, this change-speed apparatus is designed to change over a direct coupled state in which the average wheel speed of the primary drive wheels and the average wheel speed of the secondary drive wheels are substantially equal and a speed increased state in which the average wheel speed of the secondary drive wheels becomes faster than the average wheel speed of the primary drive wheels by switching ON and OFF a direct coupling clutch and a change-speed clutch. Thus, according to the front and rear wheels drive system so constructed, when turning a tight corner in the four-wheel drive running conditions, the occurrence of the tight corner braking phenomenon is prevented by putting the secondary drive wheels in the speed increased state using the change-speed apparatus.
In the change-speed apparatus of the front and rear wheels drive system, since at least two hydraulic or electromagnetic actuators are needed as a power source for operating the direct coupling clutch and the change-speed clutch, there is caused a defect that the size and weight of the apparatus are increased.
Then, an object of the invention is to provide a vehicle front and rear wheels drive system and a clutch changeover method which can enable the changeover of a change-speed apparatus in the vehicle front and rear wheels drive system using a single actuator to thereby reduce the overall size and weight of the change-speed apparatus.
With a view to attaining the object, according to a first aspect of the invention, there is provided a vehicle front and rear wheels drive system having along a power transmission path extending from primary drive wheels to secondary drive wheels a speed-changeable change-speed apparatus having a composite planetary gear device provided in a casing fixed to a vehicle body, the vehicle front and rear wheels drive system being characterized in that changeover clutch discs for, respectively, a direct coupling clutch and a change-speed clutch are disposed on a movable element provided concentrically with an input shaft or an output shaft of the change-speed apparatus to which power is transmitted from the primary drive wheels, adapted to move along the direction of the input shaft or the output shaft and meshing with a carrier of the composite planetary gear device in such a manner as not to rotate relative to the carrier, in that an elastic body and an actuator both for changing over the state of the direct coupling clutch or said change-speed clutch between engagement and disengagement are disposed such that operating directions of the elastic body and the actuator are opposed to each other in the direction of the input shaft or the output shaft, in that the elastic body brings one of the direct coupling clutch and the change-speed clutch into engagement by moving the movable element by virtue of a biasing force thereof, and in that after having released the engagement of the one clutch by moving the movable element against the biasing force of the elastic body by virtue of the thrust thereof, the actuator brings the other clutch into engagement.
Here, the xe2x80x9cprimary drive wheelsxe2x80x9d means drive wheels to which power is directly transmitted from a power source, while the xe2x80x9csecondary drive wheelsxe2x80x9d means drive wheels to which power from the power source is transmitted via torque transmission clutches which can each control the torque transmission amount thereof. In addition, the xe2x80x9celastic bodyxe2x80x9d is something like, for example, a coned disc spring, a coil spring, a plate spring, a rubber or the like. Furthermore, it goes without saying that the material of the elastic body may be changed as required by selecting a suitable one from meal, plastic, rubber and the like. Moreover, xe2x80x9cconcentricallyxe2x80x9d includes in its meaning the layout configuration of the direct coupling clutch and the change-speed clutch as will be described later.
According to the first aspect of the invention, the direct coupling clutch is kept engaged normally by moving the movable element in one direction by virtue of the biasing force of the elastic body. Then, when the actuator is actuated and the movable element is moved in the other direction against the biasing force of the elastic body by virtue of the thrust thereof the engagement of the direct coupling clutch is released. Thereafter, when the movable element is moved further in the other direction the change-speed clutch is brought into engagement. In addition, when halting the operation the actuator, since the movable clutch is allowed to move again in the one direction by virtue of the biasing force of the elastic body, the direct coupling clutch is brought into engagement again after the engagement of the changeover disc clutches is released.
According to a second aspect of the invention, there is provided a vehicle front and rear wheels drive system as set forth in the first aspect of the invention, wherein the changeover clutch discs, respectively, for the direct coupling clutch and the change-speed clutch are disposed in two stages in a radial direction across the movable element, and wherein the changeover clutch discs situated on an inner circumferential side engage with clutch discs disposed on either the input shaft or the output shaft, while the changeover clutch discs disposed on an outer circumferential side engage with clutch discs disposed on the casing.
According to the second aspect of the invention, in addition to the function provided in the first aspect of the invention, for example, by moving the movable element in the one direction by virtue of the biasing force of the elastic body the changeover clutch discs disposed on the movable element for the direct coupling clutch are allowed to normally engage with the clutch discs disposed on the input shaft. As this occurs, since the input shaft rotates together with the movable element, the power transmitted from the input shaft is outputted via the carrier of the composite planetary gear apparatus which meshes with the movable element in such a manner as not to rotate relative to the movable element. Then, when the actuator is actuated to move the movable element in the other direction against the biasing force of the elastic body by virtue of the thrust thereof the changeover clutch discs of the direct coupling clutch move apart from the clutch discs disposed on the input shaft, whereby the engagement of the direct coupling clutch is released. Thereafter, when the movable element is moved further in the other direction the changeover clutch discs of the change-speed clutch disposed on the movable element are brought into engagement with clutch discs disposed on the casing fixed to the vehicle body, whereby the change-speed clutch is brought into engagement. Thus, the input shaft rotates independently from the movable element, and in addition, the movable element that is then rotating is fixed to the casing in association with the engagement of the change-speed clutch, whereby the rotation of the carrier is halted. As this occurs, the power transmitted from the input shaft is outputted via the respective gears of the composite planetary gear device. Then, when the actuator halts the operation since the movable element moves in the one direction again by virtue of the biasing force of the elastic body, the direct coupling clutch is brought into engagement again after the engagement of the change-speed clutch has been released.
According to a third aspect of the invention, there is provided a clutch changeover method for changing over clutches in a change-speed apparatus having a direct coupling clutch and a change-speed clutch which are disposed on a movable element provided concentrically with an input shaft or an output shaft to which power is transmitted and adapted to move along the direction of the input shaft or the output shaft, and an elastic body and an actuator both for changing over the state of the direct coupling clutch or the change-speed clutch between engagement and disengagement which are disposed such that operating directions of the elastic body and the actuator are opposed to each other in the direction of the input shaft or the output shaft, the clutch changeover method comprising keeping one of the direct coupling clutch and the change-speed clutch in engagement by normally moving the movable element in one direction by virtue of a biasing force of the elastic body, and by actuating the actuator, after having released the engagement of the one clutch by moving the movable element against the biasing force of the elastic body by virtue of the thrust of the actuator so actuated, bringing the other clutch into engagement.
According to the third aspect of the invention, for example, the engagement of the direct coupling clutch, which is kept in a normally coupled state by virtue of the biasing force of the elastic body is released via the movable element by actuating the actuator. Thereafter, when the movable element is moved further in the other direction the change-speed clutch is then brought into engagement. In addition, when the operation of the actuator stops in this condition the movable element moves in the one direction by virtue of the biasing force of the elastic body, whereby the engagement of the change-speed clutch is released, and thereafter, the direct coupling clutch is brought into engagement again.